1. Field of Invention
The present invention relates to the field of light emitting devices. More specifically, the invention relates to electrical pumped organic laser devices.
2. Description of Prior Art
There has been much interest in the realization of electrically pumped organic laser devices (OLDs) as known in the art, since lasing in optically pumped organic thin films has been demonstrated using a variety of optical resonators, including optical waveguides, planar microcavities, and distributed feedback (DFB) structures (N. Tessler et al., Nature 382, 695 (1996); Ruidong Xia et al., Org. Electron. 4, 165 (2003); M. Reufer et al., Appl. Phys. Lett. 84, 3262 (2004), each of which is incorporated herein by reference). As used herein, the term “organic” includes of polymers, small molecular weight organic materials, and other kinds of organic light emitting materials that can be used to fabricate organic opto-electronic devices.
As laser gain media, organic materials are intrinsically quasi-four-level systems. They are low cost materials that have high fluorescent quantum efficiencies and low absorption losses. Optically pumped stimulated emission, gain, and lasing have been observed in various kinds of organic materials with emission wavelengths spanning the visible spectrum. It also shows that the lasing threshold, emission wavelength, output power of organic lasers have more temperature stable than conventional inorganic laser diodes. (V. G. Kozlov et al., J. Appl. Phys, 84, 4096 (1998), incorporated herein by reference). Therefore, OLDs are believed to be novel visible lasers, which show a number of applications in display, optical storage, optical communications et al.
There are some patents related to organic lasers. (U.S. Pat. No. 6,498,802, No. 6,160,828, No. 7,242,703, No. 6,853,660, No. 6,674,776, No. 5,881,089, No. 6,996,146, each of which is incorporated herein by reference). However, efforts to make electrically pumped OLDs have been unsuccessful up to now. There are still challenges to overcome such as low carrier mobility of organic films and much higher optical loss associated with electrical pumping. The low carrier mobility of organic films makes OLDs hard to achieve high current densities required, and limits the thickness of organic films to thin layers, which resulting in high optical losses in OLDs with waveguide and DFB structures. (V. G. Kozlov et al., J. Appl. Phys, 84, 4096 (1998); McGehee et al., Adv. Mater. 12, 1655 (2000), each of which is incorporated herein by reference). Recently, very low laser thresholds have been achieved in organic waveguide and microcavity structures under optical pumping, which means that electrically pumped OLDs can operate at a low current density that matching low carrier mobility of organic films. (T. W. Lee et al., J. Appl. Phys. 93, 1367 (2003); M. Berggren et al., Nature 389, 466 (1997); X. Liu et al., Appl. Phys. Lett. 84, 2727 (2004), each of which is incorporated herein by reference). Therefore, to develop an effective electrically pumped OLD, the key problem at present is how to lower the optical loss of the laser structure. (S. Lattante et al., Appl. Phys. Lett. 89, 031108 (2006); P. Görrn et al., Appl. Phys. Lett. 91, 041113 (2007), each of which is incorporated herein by reference).
One of the effective structures for electrically pumped OLD is microcavity that are formed by depositing one or multi-layer organic materials between two mirrors (either dielectric stacks or metallic) separated by a few hundred nanometers. To achieve a low threshold current, microcavity OLDs typically utilize short gain region. With such a thin gain region, microcavity OLDs have a very small single pass optical gain, thereby requiring high reflectivities equal to or greater than 97% at the interfaces between organic layers and electrodes to achieve lasing. In an electrically pumped OLD, electron carriers and hole carriers need to be injected from two electrodes, generally from an anode and a cathode, respectively. How to lower electron-injection barrier at cathode/organic interface, and hole-injection barrier at anode/organic interface is one of the critical issues for OLDs. Anode materials should employ high work function transparent conducting oxides (TCOs) such as indium tin oxide (ITO) for good hole-injection. Cathode materials should use low work function metals such as aluminum (Al), lithium (Li) or double layer structure such as Al/ITO for good electron-injection. Metal mirrors are frequently used in microcavities as they can provide suitable reflectance and excellent electrical contact. As known in the art, most thick metal mirrors have a reflectance less than 96%. However, to develop a microcavity OLD, it is essential to employ two highly reflective mirrors to construct a high quality factor (high Q) cavity. Therefore, distributed Bragg reflectors (DBRs) are the best choice among highly reflective mirrors for microcavity OLDs. In microcavity OLDs, electrodes are essentially disposed between organic layers and mirrors. In order to lower the optical absorption, microcavity OLDs should use thin and transparent electrodes. ITO film, a familiar anode, averagely has a transmittance of 85% and an absorptance of 0.5% at the thickness of 30 nm. The familiar cathode, Al metal film with the thickness of 10 nm, averagely has a transmittance of 42% and an absorptance of 37% in the visible spectra region.
To realize low loss microcavity OLDs, one challenge is how to obtain an adequate electrical contact and a reflectance equal to or higher than 97% at organic-electrode interface, especially at organic-metal interface. Another challenge is how to lower electrode-induced optical loss. There is a need, therefore for an alternative microcavity OLD that avoids these limitations.